Thermal transfer system for fired ceramic decals

Abstract

A process for preparing a ceramic decal in which a printed image is applied to a backing sheet, and a covercoating is then applied to the printed substrate. A digitally printed ceramic colorant image is applied to the backing sheet; metal oxide colorant with a refractive index of greater than about 1.6 is used as the colorant. Thereafter, the printed image is covercoated with an overcoat containing frit and binder. The total amount of frit applied in the process divided by the total amount of colorant used is at least 2, and the frit used has a melting temperature of at least 550 degrees Centigrade.

We claim:  
     
       1. A process for preparing a ceramic decal, comprising the steps of sequentially: 
       (a) preparing a first printed image substrate, by a process comprising the steps of applying to a backing sheet a digitally printed ceramic colorant image comprised of a colorant composition, wherein:  
       1. said colorant composition is comprised of metal oxide pigment with a refractive index greater than about 1.4,  
       2. said colorant composition is comprised of a multiplicity of metal oxide pigment particles, at least about 90 weight percent of which are within the range of about 0.2 to about 20 microns, and  
       3. said colorant composition is comprised of a first solid carbonaceous binder, and  
       4. said digitally printed ceramic colorant image comprises a first surface,  
       (b) digitally printing to said first surface of said first printed ceramic colorant image a flux covercoat comprised of a first mixture comprised of a first frit and a second solid carbonaceous binder, wherein:  
       1. said first frit has a melting temperature of at least about 550 degrees Centigrade,  
       2. said flux covercoat is printed over at least all of said first surface of said digitally printed ceramic colorant image, and  
       3. the total amount of frit applied to said backing sheet is at least 2 times as great as the total amount of colorant applied to said backing sheet.  
     
     
       2. The process as recited in  claim 1 , wherein said digital printing is thermal transfer printing. 
     
     
       3. The process as recited in  claim 1 , wherein said colorant composition is comprised of less than about 5 weight percent of frit. 
     
     
       4. The process as recited in  claim 3 , further comprising the step of preparing said backing sheet by a process comprising the steps of applying to a backing sheet a second mixture comprised of a second frit and a third solid carbonaceous binder, wherein said second frit has a melting temperature of at least about 550 degrees Centigrade. 
     
     
       5. The process as recited in  claim 4 , wherein: 
       (a) said second mixture is applied to said backing sheet at coverage of at least about 10 grams per square meter,  
       (b) said second frit comprises at least about 25 weight percent of said second mixture of said second frit and said third solid carbonaceous binder,  
       (c) said flux covercoat is applied to said first printed ceramic colorant image at coverage of at least 2 grams per square meter,  
       (d) said flux covercoat is comprised of at least about 25 weight percent of said second frit, provided that the total amount of frit applied to said backing sheet is at least about 4 times as great as the total amount of colorant applied to said backing sheet.  
     
     
       6. The process as recited in  claim 5 , wherein each of said first carbonaceous binder, said second carbonaceous binder, and said third carbonaceous binder is comprised of less than about 15 weight percent of liquid. 
     
     
       7. The process are recited in  claim 6 , wherein at least about 50 weight percent of said total amount of frit applied to said backing sheet is applied as said second frit. 
     
     
       8. The process as recited in  claim 7 , wherein each of said first frit and said second frit has a particle size distribution such that at least about 90 percent of the particles in such frit are smaller than about 5 microns. 
     
     
       9. The process as recited in  claim 8 , wherein each of said first frit and said second frit is comprised of at least about 5 weight percent of silica. 
     
     
       10. The process as recited in  claim 9 , wherein said second mixture is comprised of from about 35 to about 85 weight percent of said second frit. 
     
     
       11. The process as recited in  claim 10 , wherein said second mixture is comprised of from about 15 to about 35 weight percent of said third solid carbonaceous binder. 
     
     
       12. The process as recited in  claim 11 , wherein said second mixture is comprised of from about 5 to about 20 weight percent of wax. 
     
     
       13. The process as recited in  claim 12 , wherein said second mixture is comprised of from about 1 to about 15 weight percent of plasticizing agent. 
     
     
       14. The process as recited in  claim 5 , further comprising the step of printing an opacifying agent over said backing sheet prior to the time said digitally printed ceramic colorant image is applied to said backing sheet. 
     
     
       15. The process as recited in  claim 14 , wherein said opacifying agent has a melting temperature of at least about 1200 degrees Centigrade. 
     
     
       16. The process as recited in  claim 15 , wherein said opacifying agent has a refractive index greater than 2.0. 
     
     
       17. The process as recited in  claim 16 , further comprising the step of printing a third mixture of a third frit and a fourth solid carbonaceous binder over said opacifying agent prior to the time said digitally printed ceramic colorant image is applied to said backing sheet. 
     
     
       18. The process as recited in  claim 5 , wherein said flux covercoat is substantially insoluble in water. 
     
     
       19. The process as recited in  claim 18 , wherein said flux covercoat is applied at a coating weight of at least 5 grams per square meter. 
     
     
       20. The process as recited in  claim 19 , wherein said flux covercoat is comprised of flux overcoat binder. 
     
     
       21. The process as recited in  claim 18 , wherein said flux covercoat has an elongation before break of greater than 5 percent.